During transmission of an information signal from a transmitter to a receiver in a communications system, the information signal typically modulates a carrier signal. The information signal may modulate the carrier signal using a wide variety of methods, such as amplitude or frequency modulation. Although the carrier signal is modulated, a phase error component is generally introduced during transmission from the transmitter to the receiver. The phase error component is manifested as an unwanted low frequency signal which distorts the modulated information signal. The phase error is typically a result of non-linearities inherent within either the transmitter or receiver equipment, and atmospheric conditions such as cloud cover.
To correct a modulating information signal which is distorted by a phase error component, the phase error component is typically removed from the information signal using a feedback loop in analog circuitry. Digital solutions used to remove the phase error component may also be implemented. However, digital solutions require the extensive use of memory accesses and interpolation. Therefore, digital phase error correction circuits have been extremely costly to implement in a receiver system.
In an amplitude modulated (AM) stereo system, the amplitude of the carrier signal is typically modulated by the information signal such that a substantial amount of information may be transmitted in a relatively small band of frequencies. As well, stereo information associated with the transmitted signal may also be transmitted within the frequency band. Several systems for transmission and reception of AM stereo information have been developed through industry use. Each system implements a method for providing two audio channels within a predetermined band of frequencies with high quality stero sound and very little interference. However, one of the standards, an AM stereo system which uses quadrature amplitude modulation, is used most often and is, therefore, a de facto industry standard.
An industry standard AM stereo system licensed by Motorola, Inc., under the trademark "C-QUAM" is referred to as a Compatible Quadrature Amplitude Modulation stero system. The "C-QUAM" stereo system typically provides stereophonic information using amplitude modulation for a main information signal, and a quadrature type of phase modulation for a stereo information signal. Quadrature phase modulation is used to separate a composite of a left channel (L) and a right channel (R) of the stereo information signal, and a difference between the left and the right channels, by a phase angle of 90 degrees for transmission. During transmission, an oscillator signal is modulated with the composite of the left channel and the right channel of the stereo information signal, and a quadrature carrier signal is modulated with the difference between the left channel and the right channel. Together, the information carrier signal and the quadrature carrier signal provide a resultant signal. The resultant signal is then passed through a limiter which removes all amplitude modulated components to provide a limited resultant signal. The limited resultant signal is then amplified and input to the transmitter as a carrier signal. The composite of the left and right channels is provided as an audio input to the transmitter. The transmitter then provides the composite of the left and right channels at a carrier frequency with a phase modulation, where the carrier frequency is equal to the oscillator input of the transmitter. A signal broadcast by the transmitter must then be separated into the composite of and the difference between the left channel and the right channel of the stereo information signal at a receiver.
In the "C-QUAM" analog stereo receiver, stereophonic components are extracted from a broadcast signal using standard analog circuits. Typically, the broadcast signal is converted to a pure quadrature information signal, and a quadrature demodulator is then used to extract both the composite and difference of the left and the right channels of the broadcast signal. Before the broadcast signal is input to the quadrature demodulator, the signal must be converted to an original transmitted quadrature signal that contains phase modulation components. To convert the broadcast signal to its original form, the signal must be demodulated with both an envelope detector and with a sideband detector. The signals provided by both the envelope detector and the sideband detector are then compared and the resultant error signal gain modulates the inputs of the sideband detector. For further information on the operation of a "C-QUAM" encoder and receiver, refer to "Introduction to the Motorola "C-QUAM" AM Stereo System" published by Motorola, Inc. in 1985.
Although an analog solution adequately demodulates the broadcast signal and subsequently separates the broadcast signal into a left and a right signal, the signal quality of the broadcast signal is limited by the nature of the analog solution. Particularly, the envelope detector used in the receiver described above is inherently prone to produce various types of distortion, thereby limiting the audio quality of the AM stereo system. As well, in specialized applications such as an automobile, a small acoustic chamber and a highly variable background noise signal adversely affect the audio quality of any stereo signal. Acoustic equalization may be used to compensate for the small acoustic chamber and adaptive noise suppression may be provided to compensate for the background noise. However, both acoustic equalization and noise suppression techniques are very difficult to implement in an analog system.
Additionally, separate receivers must be used for each type of stereo format and function. For example, separate receivers are needed for FM and AM stereo formats. Therefore, a stereo system which requires both FM and AM stereo formats must have two or more receivers depending on the specifications of the system.
Therefore, a need exists for an AM stereo receiver which demodulates a broadcast signal to produce a high quality stereo signal. The stereo receiver should also remove any phase error components which might distort the broadcast signal in a timely and economical manner. A receiver which can support several stereo functions, such as AM and FM stereo is also needed. Additionally, it is desirable to include equalization and adaptive noise suppression techniques in a receiver to respectively compensate for a small acoustic chamber and variable background noise. Other known sound enhancements and effects, such as reverberation, are also desired features to include in any stereo system.